The present invention relates to electrical switching devices, and more particularly, relates to the development and operation of a load-break switch which includes a vacuum interrupter.
Generally, vacuum interrupters used for electrical switching devices, particularly in the 6 to 35 kV voltage range, include an insulating, tubular housing having two conductive end caps. Two coaxially opposing contact studs are introduced into the housing through the conductive end caps. One contact stud is directly connected in a vacuum-tight manner to one end cap and the other contact stud is connected via a bellows to the other end cap. The contact studs are provided with contact surfaces configured inside the housing. U.S. Pat. No. 3,590,197 discloses an example of such a configuration. The contact surfaces are formed from contact pieces made of a special material and with a special constructive design. Such a design is disclosed in German Published Patent Application 26 38 700. Contact pieces of iron-containing material such as Maraging steel or rust-free chromium-nickel steels have proven to be suitable as disclosed German Published Patent Application 23 08 913 and British Patent 1 480 285. It is known that the end faces of the contact studs themselves form the contact surfaces in a vacuum interrupter having contact studs of a copper alloy comprising 0.003% to 0.5% boron and preferably 0.1% to 1.00% bismuth as disclosed in British Printed Patent 1 309 197.
Vacuum interrupters can be used for circuit-breakers, and for load-break switches and contactors. In load-break switches, the vacuum interrupters can only be loaded with the maximum allowable continuous-load current. Therefore, in this type of application, the qualities of these interrupters cannot be fully utilized. To better utilize the breaking capacity of the interrupters used in load-break switches, a first switch is configured in series with the vacuum interrupter and a second switch is configured parallel to this series connection. The parallel configured switch is designed for the full load-break-switch rated current. When the load-break switch is disconnected, this parallel configured switch is first opened. Then, the vacuum interrupter executes an arc-breaking. Finally, the series-connected switch opens forming a visible separation point. When the load-break switch is switched on, the vacuum interrupter is connected after the series-configured switch is switched on, or only after both switches are switched on. German Published Paten Application 25 22 525 discloses such a vacuum interrupter switch configuration.
There is a need for a load-break switch which includes a vacuum interrupter that is essentially tailored only to the breaking operation for a load current.